/* database.cc - The database interfaces of the notmuch mail library
 *
 * Copyright © 2009 Carl Worth
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see http://www.gnu.org/licenses/ .
 *
 * Author: Carl Worth <cworth@cworth.org>
 */

#include "database-private.h"

#include <iostream>

#include <xapian.h>

#include <glib.h> /* g_free, GPtrArray, GHashTable */

using namespace std;

#define ARRAY_SIZE(arr) (sizeof (arr) / sizeof (arr[0]))

typedef struct {
    const char *name;
    const char *prefix;
} prefix_t;

/* Here's the current schema for our database:
 *
 * We currently have two different types of documents: mail and timestamps.
 *
 * Mail document
 * -------------
 * A mail document is associated with a particular email message file
 * on disk. It is indexed with the following prefixed terms which the
 * database uses to construct threads, etc.:
 *
 *    Single terms of given prefix:
 *
 *	type:	mail
 *
 *	id:	Unique ID of mail, (from Message-ID header or generated
 *		as "notmuch-sha1-<sha1_sum_of_entire_file>.
 *
 *	thread:	The ID of the thread to which the mail belongs
 *
 *	replyto: The ID from the In-Reply-To header of the mail (if any).
 *
 *    Multiple terms of given prefix:
 *
 *	reference: All message IDs from In-Reply-To and Re ferences
 *		   headers in the message.
 *
 *	tag:	   Any tags associated with this message by the user.
 *
 *    A mail document also has two values:
 *
 *	TIMESTAMP:	The time_t value corresponding to the message's
 *			Date header.
 *
 *	MESSAGE_ID:	The unique ID of the mail mess (see "id" above)
 *
 * In addition, terms from the content of the message are added with "from",
 * "to", "attachment", "subject", and "folder" prefixes for use by the user in
 * searching. But the database doesn't really care itself about any of these.
 *
 * Timestamp document
 * ------------------
 * A timestamp document is used by a client of the notmuch library to
 * maintain data necessary to allow for efficient polling of mail
 * directories. The notmuch library does no interpretation of
 * timestamps, but merely allows the user to store and retrieve
 * timestamps as name/value pairs.
 *
 * The timestamp document is indexed with a single prefixed term:
 *
 *	timestamp:	The user's key value (likely a directory name)
 *
 * and has a single value:
 *
 *	TIMESTAMP:	The time_t value from the user.
 */

/* With these prefix values we follow the conventions published here:
 *
 * http://xapian.org/docs/omega/termprefixes.html
 *
 * as much as makes sense. Note that I took some liberty in matching
 * the reserved prefix values to notmuch concepts, (for example, 'G'
 * is documented as "newsGroup (or similar entity - e.g. a web forum
 * name)", for which I think the thread is the closest analogue in
 * notmuch. This in spite of the fact that we will eventually be
 * storing mailing-list messages where 'G' for "mailing list name"
 * might be even a closer analogue. I'm treating the single-character
 * prefixes preferentially for core notmuch concepts (which will be
 * nearly universal to all mail messages).
 */

prefix_t BOOLEAN_PREFIX_INTERNAL[] = {
    { "type", "T" },
    { "reference", "XREFERENCE" },
    { "replyto", "XREPLYTO" },
    { "timestamp", "XTIMESTAMP" },
};

prefix_t BOOLEAN_PREFIX_EXTERNAL[] = {
    { "thread", "G" },
    { "tag", "K" },
    { "id", "Q" }
};

prefix_t PROBABILISTIC_PREFIX[]= {
    { "from", "XFROM" },
    { "to", "XTO" },
    { "attachment", "XATTACHMENT" },
    { "subject", "XSUBJECT"},
    { "folder", "XFOLDER"}
};

int
_internal_error (const char *format, ...)
{
    va_list va_args;

    va_start (va_args, format);

    fprintf (stderr, "Internal error: ");
    vfprintf (stderr, format, va_args);

    exit (1);

    return 1;
}

const char *
_find_prefix (const char *name)
{
    unsigned int i;

    for (i = 0; i < ARRAY_SIZE (BOOLEAN_PREFIX_INTERNAL); i++) {
	if (strcmp (name, BOOLEAN_PREFIX_INTERNAL[i].name) == 0)
	    return BOOLEAN_PREFIX_INTERNAL[i].prefix;
    }

    for (i = 0; i < ARRAY_SIZE (BOOLEAN_PREFIX_EXTERNAL); i++) {
	if (strcmp (name, BOOLEAN_PREFIX_EXTERNAL[i].name) == 0)
	    return BOOLEAN_PREFIX_EXTERNAL[i].prefix;
    }

    for (i = 0; i < ARRAY_SIZE (PROBABILISTIC_PREFIX); i++) {
	if (strcmp (name, PROBABILISTIC_PREFIX[i].name) == 0)
	    return PROBABILISTIC_PREFIX[i].prefix;
    }

    INTERNAL_ERROR ("No prefix exists for '%s'\n", name);

    return "";
}

const char *
notmuch_status_to_string (notmuch_status_t status)
{
    switch (status) {
    case NOTMUCH_STATUS_SUCCESS:
	return "No error occurred";
    case NOTMUCH_STATUS_OUT_OF_MEMORY:
	return "Out of memory";
    case NOTMUCH_STATUS_READONLY_DATABASE:
	return "The database is read-only";
    case NOTMUCH_STATUS_XAPIAN_EXCEPTION:
	return "A Xapian exception occurred";
    case NOTMUCH_STATUS_FILE_ERROR:
	return "Something went wrong trying to read or write a file";
    case NOTMUCH_STATUS_FILE_NOT_EMAIL:
	return "File is not an email";
    case NOTMUCH_STATUS_DUPLICATE_MESSAGE_ID:
	return "Message ID is identical to a message in database";
    case NOTMUCH_STATUS_NULL_POINTER:
	return "Erroneous NULL pointer";
    case NOTMUCH_STATUS_TAG_TOO_LONG:
	return "Tag value is too long (exceeds NOTMUCH_TAG_MAX)";
    case NOTMUCH_STATUS_UNBALANCED_FREEZE_THAW:
	return "Unbalanced number of calls to notmuch_message_freeze/thaw";
    default:
    case NOTMUCH_STATUS_LAST_STATUS:
	return "Unknown error status value";
    }
}

static void
find_doc_ids (notmuch_database_t *notmuch,
	      const char *prefix_name,
	      const char *value,
	      Xapian::PostingIterator *begin,
	      Xapian::PostingIterator *end)
{
    Xapian::PostingIterator i;
    char *term;

    term = talloc_asprintf (notmuch, "%s%s",
			    _find_prefix (prefix_name), value);

    *begin = notmuch->xapian_db->postlist_begin (term);

    *end = notmuch->xapian_db->postlist_end (term);

    talloc_free (term);
}

static notmuch_private_status_t
find_unique_doc_id (notmuch_database_t *notmuch,
		    const char *prefix_name,
		    const char *value,
		    unsigned int *doc_id)
{
    Xapian::PostingIterator i, end;

    find_doc_ids (notmuch, prefix_name, value, &i, &end);

    if (i == end) {
	*doc_id = 0;
	return NOTMUCH_PRIVATE_STATUS_NO_DOCUMENT_FOUND;
    } else {
	*doc_id = *i;
	return NOTMUCH_PRIVATE_STATUS_SUCCESS;
    }
}

static Xapian::Document
find_document_for_doc_id (notmuch_database_t *notmuch, unsigned doc_id)
{
    return notmuch->xapian_db->get_document (doc_id);
}

static notmuch_private_status_t
find_unique_document (notmuch_database_t *notmuch,
		      const char *prefix_name,
		      const char *value,
		      Xapian::Document *document,
		      unsigned int *doc_id)
{
    notmuch_private_status_t status;

    status = find_unique_doc_id (notmuch, prefix_name, value, doc_id);

    if (status) {
	*document = Xapian::Document ();
	return status;
    }

    *document = find_document_for_doc_id (notmuch, *doc_id);
    return NOTMUCH_PRIVATE_STATUS_SUCCESS;
}

notmuch_message_t *
notmuch_database_find_message (notmuch_database_t *notmuch,
			       const char *message_id)
{
    notmuch_private_status_t status;
    unsigned int doc_id;

    status = find_unique_doc_id (notmuch, "id", message_id, &doc_id);

    if (status == NOTMUCH_PRIVATE_STATUS_NO_DOCUMENT_FOUND)
	return NULL;

    return _notmuch_message_create (notmuch, notmuch, doc_id, NULL);
}

/* Advance 'str' past any whitespace or RFC 822 comments. A comment is
 * a (potentially nested) parenthesized sequence with '\' used to
 * escape any character (including parentheses).
 *
 * If the sequence to be skipped continues to the end of the string,
 * then 'str' will be left pointing at the final terminating '\0'
 * character.
 */
static void
skip_space_and_comments (const char **str)
{
    const char *s;

    s = *str;
    while (*s && (isspace (*s) || *s == '(')) {
	while (*s && isspace (*s))
	    s++;
	if (*s == '(') {
	    int nesting = 1;
	    s++;
	    while (*s && nesting) {
		if (*s == '(') {
		    nesting++;
		} else if (*s == ')') {
		    nesting--;
		} else if (*s == '\\') {
		    if (*(s+1))
			s++;
		}
		s++;
	    }
	}
    }

    *str = s;
}

/* Parse an RFC 822 message-id, discarding whitespace, any RFC 822
 * comments, and the '<' and '>' delimeters.
 *
 * If not NULL, then *next will be made to point to the first character
 * not parsed, (possibly pointing to the final '\0' terminator.
 *
 * Returns a newly talloc'ed string belonging to 'ctx'.
 *
 * Returns NULL if there is any error parsing the message-id. */
static char *
_parse_message_id (void *ctx, const char *message_id, const char **next)
{
    const char *s, *end;
    char *result;

    if (message_id == NULL || *message_id == '\0')
	return NULL;

    s = message_id;

    skip_space_and_comments (&s);

    /* Skip any unstructured text as well. */
    while (*s && *s != '<')
	s++;

    if (*s == '<') {
	s++;
    } else {
	if (next)
	    *next = s;
	return NULL;
    }

    skip_space_and_comments (&s);

    end = s;
    while (*end && *end != '>')
	end++;
    if (next) {
	if (*end)
	    *next = end + 1;
	else
	    *next = end;
    }

    if (end > s && *end == '>')
	end--;
    if (end <= s)
	return NULL;

    result = talloc_strndup (ctx, s, end - s + 1);

    /* Finally, collapse any whitespace that is within the message-id
     * itself. */
    {
	char *r;
	int len;

	for (r = result, len = strlen (r); *r; r++, len--)
	    if (*r == ' ' || *r == '\t')
		memmove (r, r+1, len);
    }

    return result;
}

/* Parse a References header value, putting a (talloc'ed under 'ctx')
 * copy of each referenced message-id into 'hash'.
 *
 * We explicitly avoid including any reference identical to
 * 'message_id' in the result (to avoid mass confusion when a single
 * message references itself cyclically---and yes, mail messages are
 * not infrequent in the wild that do this---don't ask me why).
*/
static void
parse_references (void *ctx,
		  const char *message_id,
		  GHashTable *hash,
		  const char *refs)
{
    char *ref;

    if (refs == NULL || *refs == '\0')
	return;

    while (*refs) {
	ref = _parse_message_id (ctx, refs, &refs);

	if (ref && strcmp (ref, message_id))
	    g_hash_table_insert (hash, ref, NULL);
    }
}

notmuch_database_t *
notmuch_database_create (const char *path)
{
    notmuch_database_t *notmuch = NULL;
    char *notmuch_path = NULL;
    struct stat st;
    int err;

    if (path == NULL) {
	fprintf (stderr, "Error: Cannot create a database for a NULL path.\n");
	goto DONE;
    }

    err = stat (path, &st);
    if (err) {
	fprintf (stderr, "Error: Cannot create database at %s: %s.\n",
		 path, strerror (errno));
	goto DONE;
    }

    if (! S_ISDIR (st.st_mode)) {
	fprintf (stderr, "Error: Cannot create database at %s: Not a directory.\n",
		 path);
	goto DONE;
    }

    notmuch_path = talloc_asprintf (NULL, "%s/%s", path, ".notmuch");

    err = mkdir (notmuch_path, 0755);

    if (err) {
	fprintf (stderr, "Error: Cannot create directory %s: %s.\n",
		 notmuch_path, strerror (errno));
	goto DONE;
    }

    notmuch = notmuch_database_open (path,
				     NOTMUCH_DATABASE_MODE_READ_WRITE);

  DONE:
    if (notmuch_path)
	talloc_free (notmuch_path);

    return notmuch;
}

notmuch_database_t *
notmuch_database_open (const char *path,
		       notmuch_database_mode_t mode)
{
    notmuch_database_t *notmuch = NULL;
    char *notmuch_path = NULL, *xapian_path = NULL;
    struct stat st;
    int err;
    unsigned int i;

    if (asprintf (&notmuch_path, "%s/%s", path, ".notmuch") == -1) {
	notmuch_path = NULL;
	fprintf (stderr, "Out of memory\n");
	goto DONE;
    }

    err = stat (notmuch_path, &st);
    if (err) {
	fprintf (stderr, "Error opening database at %s: %s\n",
		 notmuch_path, strerror (errno));
	goto DONE;
    }

    if (asprintf (&xapian_path, "%s/%s", notmuch_path, "xapian") == -1) {
	xapian_path = NULL;
	fprintf (stderr, "Out of memory\n");
	goto DONE;
    }

    notmuch = talloc (NULL, notmuch_database_t);
    notmuch->exception_reported = FALSE;
    notmuch->path = talloc_strdup (notmuch, path);

    if (notmuch->path[strlen (notmuch->path) - 1] == '/')
	notmuch->path[strlen (notmuch->path) - 1] = '\0';

    notmuch->mode = mode;
    try {
	if (mode == NOTMUCH_DATABASE_MODE_READ_WRITE) {
	    notmuch->xapian_db = new Xapian::WritableDatabase (xapian_path,
							       Xapian::DB_CREATE_OR_OPEN);
	} else {
	    notmuch->xapian_db = new Xapian::Database (xapian_path);
	}
	notmuch->query_parser = new Xapian::QueryParser;
	notmuch->term_gen = new Xapian::TermGenerator;
	notmuch->term_gen->set_stemmer (Xapian::Stem ("english"));
	notmuch->value_range_processor = new Xapian::NumberValueRangeProcessor (NOTMUCH_VALUE_TIMESTAMP);

	notmuch->query_parser->set_default_op (Xapian::Query::OP_AND);
	notmuch->query_parser->set_database (*notmuch->xapian_db);
	notmuch->query_parser->set_stemmer (Xapian::Stem ("english"));
	notmuch->query_parser->set_stemming_strategy (Xapian::QueryParser::STEM_SOME);
	notmuch->query_parser->add_valuerangeprocessor (notmuch->value_range_processor);

	for (i = 0; i < ARRAY_SIZE (BOOLEAN_PREFIX_EXTERNAL); i++) {
	    prefix_t *prefix = &BOOLEAN_PREFIX_EXTERNAL[i];
	    notmuch->query_parser->add_boolean_prefix (prefix->name,
						       prefix->prefix);
	}

	for (i = 0; i < ARRAY_SIZE (PROBABILISTIC_PREFIX); i++) {
	    prefix_t *prefix = &PROBABILISTIC_PREFIX[i];
	    notmuch->query_parser->add_prefix (prefix->name, prefix->prefix);
	}
    } catch (const Xapian::Error &error) {
	fprintf (stderr, "A Xapian exception occurred opening database: %s\n",
		 error.get_msg().c_str());
	notmuch = NULL;
    }

  DONE:
    if (notmuch_path)
	free (notmuch_path);
    if (xapian_path)
	free (xapian_path);

    return notmuch;
}

void
notmuch_database_close (notmuch_database_t *notmuch)
{
    try {
	if (notmuch->mode == NOTMUCH_DATABASE_MODE_READ_WRITE)
	    (static_cast <Xapian::WritableDatabase *> (notmuch->xapian_db))->flush ();
    } catch (const Xapian::Error &error) {
	if (! notmuch->exception_reported) {
	    fprintf (stderr, "Error: A Xapian exception occurred flushing database: %s\n",
		     error.get_msg().c_str());
	}
    }

    delete notmuch->term_gen;
    delete notmuch->query_parser;
    delete notmuch->xapian_db;
    delete notmuch->value_range_processor;
    talloc_free (notmuch);
}

const char *
notmuch_database_get_path (notmuch_database_t *notmuch)
{
    return notmuch->path;
}

static notmuch_private_status_t
find_timestamp_document (notmuch_database_t *notmuch, const char *db_key,
			 Xapian::Document *doc, unsigned int *doc_id)
{
    return find_unique_document (notmuch, "timestamp", db_key, doc, doc_id);
}

/* We allow the user to use arbitrarily long keys for timestamps,
 * (they're for filesystem paths after all, which have no limit we
 * know about). But we have a term-length limit. So if we exceed that,
 * we'll use the SHA-1 of the user's key as the actual key for
 * constructing a database term.
 *
 * Caution: This function returns a newly allocated string which the
 * caller should free() when finished.
 */
static char *
timestamp_db_key (const char *key)
{
    int term_len = strlen (_find_prefix ("timestamp")) + strlen (key);

    if (term_len > NOTMUCH_TERM_MAX)
	return notmuch_sha1_of_string (key);
    else
	return strdup (key);
}

notmuch_status_t
notmuch_database_set_timestamp (notmuch_database_t *notmuch,
				const char *key, time_t timestamp)
{
    Xapian::Document doc;
    Xapian::WritableDatabase *db;
    unsigned int doc_id;
    notmuch_private_status_t status;
    notmuch_status_t ret = NOTMUCH_STATUS_SUCCESS;
    char *db_key = NULL;

    if (notmuch->mode == NOTMUCH_DATABASE_MODE_READ_ONLY) {
	fprintf (stderr, "Attempted to update a read-only database.\n");
	return NOTMUCH_STATUS_READONLY_DATABASE;
    }

    db = static_cast <Xapian::WritableDatabase *> (notmuch->xapian_db);
    db_key = timestamp_db_key (key);

    try {
	status = find_timestamp_document (notmuch, db_key, &doc, &doc_id);

	doc.add_value (NOTMUCH_VALUE_TIMESTAMP,
		       Xapian::sortable_serialise (timestamp));

	if (status == NOTMUCH_PRIVATE_STATUS_NO_DOCUMENT_FOUND) {
	    char *term = talloc_asprintf (NULL, "%s%s",
					  _find_prefix ("timestamp"), db_key);
	    doc.add_term (term);
	    talloc_free (term);

	    db->add_document (doc);
	} else {
	    db->replace_document (doc_id, doc);
	}

    } catch (const Xapian::Error &error) {
	fprintf (stderr, "A Xapian exception occurred setting timestamp: %s.\n",
		 error.get_msg().c_str());
	notmuch->exception_reported = TRUE;
	ret = NOTMUCH_STATUS_XAPIAN_EXCEPTION;
    }

    if (db_key)
	free (db_key);

    return ret;
}

time_t
notmuch_database_get_timestamp (notmuch_database_t *notmuch, const char *key)
{
    Xapian::Document doc;
    unsigned int doc_id;
    notmuch_private_status_t status;
    char *db_key = NULL;
    time_t ret = 0;

    db_key = timestamp_db_key (key);

    try {
	status = find_timestamp_document (notmuch, db_key, &doc, &doc_id);

	if (status == NOTMUCH_PRIVATE_STATUS_NO_DOCUMENT_FOUND)
	    goto DONE;

	ret =  Xapian::sortable_unserialise (doc.get_value (NOTMUCH_VALUE_TIMESTAMP));
    } catch (Xapian::Error &error) {
	ret = 0;
	goto DONE;
    }

  DONE:
    if (db_key)
	free (db_key);

    return ret;
}

/* Find the thread ID to which the message with 'message_id' belongs.
 *
 * Returns NULL if no message with message ID 'message_id' is in the
 * database.
 *
 * Otherwise, returns a newly talloced string belonging to 'ctx'.
 */
static const char *
_resolve_message_id_to_thread_id (notmuch_database_t *notmuch,
				  void *ctx,
				  const char *message_id)
{
    notmuch_message_t *message;
    const char *ret = NULL;

    message = notmuch_database_find_message (notmuch, message_id);
    if (message == NULL)
	goto DONE;

    ret = talloc_steal (ctx, notmuch_message_get_thread_id (message));

  DONE:
    if (message)
	notmuch_message_destroy (message);

    return ret;
}

static notmuch_status_t
_merge_threads (notmuch_database_t *notmuch,
		const char *winner_thread_id,
		const char *loser_thread_id)
{
    Xapian::PostingIterator loser, loser_end;
    notmuch_message_t *message = NULL;
    notmuch_private_status_t private_status;
    notmuch_status_t ret = NOTMUCH_STATUS_SUCCESS;

    find_doc_ids (notmuch, "thread", loser_thread_id, &loser, &loser_end);

    for ( ; loser != loser_end; loser++) {
	message = _notmuch_message_create (notmuch, notmuch,
					   *loser, &private_status);
	if (message == NULL) {
	    ret = COERCE_STATUS (private_status,
				 "Cannot find document for doc_id from query");
	    goto DONE;
	}

	_notmuch_message_remove_term (message, "thread", loser_thread_id);
	_notmuch_message_add_term (message, "thread", winner_thread_id);
	_notmuch_message_sync (message);

	notmuch_message_destroy (message);
	message = NULL;
    }

  DONE:
    if (message)
	notmuch_message_destroy (message);

    return ret;
}

static void
_my_talloc_free_for_g_hash (void *ptr)
{
    talloc_free (ptr);
}

static notmuch_status_t
_notmuch_database_link_message_to_parents (notmuch_database_t *notmuch,
					   notmuch_message_t *message,
					   notmuch_message_file_t *message_file,
					   const char **thread_id)
{
    GHashTable *parents = NULL;
    const char *refs, *in_reply_to, *in_reply_to_message_id;
    GList *l, *keys = NULL;
    notmuch_status_t ret = NOTMUCH_STATUS_SUCCESS;

    parents = g_hash_table_new_full (g_str_hash, g_str_equal,
				     _my_talloc_free_for_g_hash, NULL);

    refs = notmuch_message_file_get_header (message_file, "references");
    parse_references (message, notmuch_message_get_message_id (message),
		      parents, refs);

    in_reply_to = notmuch_message_file_get_header (message_file, "in-reply-to");
    parse_references (message, notmuch_message_get_message_id (message),
		      parents, in_reply_to);

    /* Carefully avoid adding any self-referential in-reply-to term. */
    in_reply_to_message_id = _parse_message_id (message, in_reply_to, NULL);
    if (in_reply_to_message_id &&
	strcmp (in_reply_to_message_id,
		notmuch_message_get_message_id (message)))
    {
	_notmuch_message_add_term (message, "replyto",
			     _parse_message_id (message, in_reply_to, NULL));
    }

    keys = g_hash_table_get_keys (parents);
    for (l = keys; l; l = l->next) {
	char *parent_message_id;
	const char *parent_thread_id;

	parent_message_id = (char *) l->data;
	parent_thread_id = _resolve_message_id_to_thread_id (notmuch,
							     message,
							     parent_message_id);

	if (parent_thread_id == NULL) {
	    _notmuch_message_add_term (message, "reference",
				       parent_message_id);
	} else {
	    if (*thread_id == NULL) {
		*thread_id = talloc_strdup (message, parent_thread_id);
		_notmuch_message_add_term (message, "thread", *thread_id);
	    } else if (strcmp (*thread_id, parent_thread_id)) {
		ret = _merge_threads (notmuch, *thread_id, parent_thread_id);
		if (ret)
		    goto DONE;
	    }
	}
    }

  DONE:
    if (keys)
	g_list_free (keys);
    if (parents)
	g_hash_table_unref (parents);

    return ret;
}

static notmuch_status_t
_notmuch_database_link_message_to_children (notmuch_database_t *notmuch,
					    notmuch_message_t *message,
					    const char **thread_id)
{
    const char *message_id = notmuch_message_get_message_id (message);
    Xapian::PostingIterator child, children_end;
    notmuch_message_t *child_message = NULL;
    const char *child_thread_id;
    notmuch_status_t ret = NOTMUCH_STATUS_SUCCESS;
    notmuch_private_status_t private_status;

    find_doc_ids (notmuch, "reference", message_id, &child, &children_end);

    for ( ; child != children_end; child++) {

	child_message = _notmuch_message_create (message, notmuch,
						 *child, &private_status);
	if (child_message == NULL) {
	    ret = COERCE_STATUS (private_status,
				 "Cannot find document for doc_id from query");
	    goto DONE;
	}

	child_thread_id = notmuch_message_get_thread_id (child_message);
	if (*thread_id == NULL) {
	    *thread_id = talloc_strdup (message, child_thread_id);
	    _notmuch_message_add_term (message, "thread", *thread_id);
	} else if (strcmp (*thread_id, child_thread_id)) {
	    _notmuch_message_remove_term (child_message, "reference",
					  message_id);
	    _notmuch_message_sync (child_message);
	    ret = _merge_threads (notmuch, *thread_id, child_thread_id);
	    if (ret)
		goto DONE;
	}

	notmuch_message_destroy (child_message);
	child_message = NULL;
    }

  DONE:
    if (child_message)
	notmuch_message_destroy (child_message);

    return ret;
}

/* Given a (mostly empty) 'message' and its corresponding
 * 'message_file' link it to existing threads in the database.
 *
 * We first look at 'message_file' and its link-relevant headers
 * (References and In-Reply-To) for message IDs. We also look in the
 * database for existing message that reference 'message'.
 *
 * The end result is to call _notmuch_message_ensure_thread_id which
 * generates a new thread ID if the message doesn't connect to any
 * existing threads.
 */
static notmuch_status_t
_notmuch_database_link_message (notmuch_database_t *notmuch,
				notmuch_message_t *message,
				notmuch_message_file_t *message_file)
{
    notmuch_status_t status;
    const char *thread_id = NULL;

    status = _notmuch_database_link_message_to_parents (notmuch, message,
							message_file,
							&thread_id);
    if (status)
	return status;

    status = _notmuch_database_link_message_to_children (notmuch, message,
							 &thread_id);
    if (status)
	return status;

    if (thread_id == NULL)
	_notmuch_message_ensure_thread_id (message);

    return NOTMUCH_STATUS_SUCCESS;
}

notmuch_status_t
notmuch_database_add_message (notmuch_database_t *notmuch,
			      const char *filename,
			      const char *folder_name,
			      notmuch_message_t **message_ret)
{
    notmuch_message_file_t *message_file;
    notmuch_message_t *message = NULL;
    notmuch_status_t ret = NOTMUCH_STATUS_SUCCESS;
    notmuch_private_status_t private_status;

    const char *date, *header;
    const char *from, *to, *subject;
    char *message_id = NULL;

    if (message_ret)
	*message_ret = NULL;

    message_file = notmuch_message_file_open (filename);
    if (message_file == NULL) {
	ret = NOTMUCH_STATUS_FILE_ERROR;
	goto DONE;
    }

    notmuch_message_file_restrict_headers (message_file,
					   "date",
					   "from",
					   "in-reply-to",
					   "message-id",
					   "references",
					   "subject",
					   "to",
					   (char *) NULL);

    try {
	/* Before we do any real work, (especially before doing a
	 * potential SHA-1 computation on the entire file's contents),
	 * let's make sure that what we're looking at looks like an
	 * actual email message.
	 */
	from = notmuch_message_file_get_header (message_file, "from");
	subject = notmuch_message_file_get_header (message_file, "subject");
	to = notmuch_message_file_get_header (message_file, "to");

	if ((from == NULL || *from == '\0') &&
	    (subject == NULL || *subject == '\0') &&
	    (to == NULL || *to == '\0'))
	{
	    ret = NOTMUCH_STATUS_FILE_NOT_EMAIL;
	    goto DONE;
	}

	/* Now that we're sure it's mail, the first order of business
	 * is to find a message ID (or else create one ourselves). */

	header = notmuch_message_file_get_header (message_file, "message-id");
	if (header && *header != '\0') {
	    message_id = _parse_message_id (message_file, header, NULL);

	    /* So the header value isn't RFC-compliant, but it's
	     * better than no message-id at all. */
	    if (message_id == NULL)
		message_id = talloc_strdup (message_file, header);

	    /* Reject a Message ID that's too long. */
	    if (message_id && strlen (message_id) + 1 > NOTMUCH_TERM_MAX) {
		talloc_free (message_id);
		message_id = NULL;
	    }
	}

	if (message_id == NULL ) {
	    /* No message-id at all, let's generate one by taking a
	     * hash over the file's contents. */
	    char *sha1 = notmuch_sha1_of_file (filename);

	    /* If that failed too, something is really wrong. Give up. */
	    if (sha1 == NULL) {
		ret = NOTMUCH_STATUS_FILE_ERROR;
		goto DONE;
	    }

	    message_id = talloc_asprintf (message_file,
					  "notmuch-sha1-%s", sha1);
	    free (sha1);
	}

	/* Now that we have a message ID, we get a message object,
	 * (which may or may not reference an existing document in the
	 * database). */

	message = _notmuch_message_create_for_message_id (notmuch,
							  message_id,
							  &private_status);

	talloc_free (message_id);

	if (message == NULL) {
	    ret = COERCE_STATUS (private_status,
				 "Unexpected status value from _notmuch_message_create_for_message_id");
	    goto DONE;
	}

	/* Is this a newly created message object? */
	if (private_status == NOTMUCH_PRIVATE_STATUS_NO_DOCUMENT_FOUND) {
	    _notmuch_message_set_filename (message, filename);
	    _notmuch_message_add_term (message, "type", "mail");
	} else {
	    ret = NOTMUCH_STATUS_DUPLICATE_MESSAGE_ID;
	    goto DONE;
	}

	ret = _notmuch_database_link_message (notmuch, message, message_file);
	if (ret)
	    goto DONE;

	date = notmuch_message_file_get_header (message_file, "date");
	_notmuch_message_set_date (message, date);

	_notmuch_message_index_file (message, filename);

        if (folder_name != NULL)
            _notmuch_message_gen_terms (message, "folder", folder_name);

	_notmuch_message_sync (message);
    } catch (const Xapian::Error &error) {
	fprintf (stderr, "A Xapian exception occurred adding message: %s.\n",
		 error.get_description().c_str());
	notmuch->exception_reported = TRUE;
	ret = NOTMUCH_STATUS_XAPIAN_EXCEPTION;
	goto DONE;
    }

  DONE:
    if (message) {
	if (ret == NOTMUCH_STATUS_SUCCESS && message_ret)
	    *message_ret = message;
	else
	    notmuch_message_destroy (message);
    }

    if (message_file)
	notmuch_message_file_close (message_file);

    return ret;
}

notmuch_tags_t *
_notmuch_convert_tags (void *ctx, Xapian::TermIterator &i,
		       Xapian::TermIterator &end)
{
    const char *prefix = _find_prefix ("tag");
    notmuch_tags_t *tags;
    std::string tag;

    /* Currently this iteration is written with the assumption that
     * "tag" has a single-character prefix. */
    assert (strlen (prefix) == 1);

    tags = _notmuch_tags_create (ctx);
    if (unlikely (tags == NULL))
	return NULL;

    i.skip_to (prefix);

    while (i != end) {
	tag = *i;

	if (tag.empty () || tag[0] != *prefix)
	    break;

	_notmuch_tags_add_tag (tags, tag.c_str () + 1);

	i++;
    }

    _notmuch_tags_prepare_iterator (tags);

    return tags;
}

notmuch_tags_t *
notmuch_database_get_all_tags (notmuch_database_t *db)
{
    Xapian::TermIterator i, end;
    i = db->xapian_db->allterms_begin();
    end = db->xapian_db->allterms_end();
    return _notmuch_convert_tags(db, i, end);
}